Static method declrations

added static to method decrlarations to avoid php warnings like this error type: [2048] call_user_func_array() expects parameter 1 to be a valid callback, non-static method Sputnik::add_download_link() should not be called statically file: H:\web-site-dev\sparkle-gear.com\wp-includes\plugin.php line: 173

Addresses #623